Common string processing functions in C language

 gets、strcpy、strncpy Function when dealing with strings

String input function gets：

char *gets(char *s);

gets Function to enter a string into a character array s in .s Is a character array or a pointer to a character array .

String string copy function strcpy、strncpy：

char *strcpy(char *s1,const char*s2);

char *strncpy(char *s1,const char*s2,size_t n);

strcpy Function will s2 Copy string from to s1 in , Include empty characters .s1 Is a character array or a pointer to a character array ,s2 character constant 、 A pointer to an array of characters .strncpy take s2 Not more than n Copy characters of characters to s1 in , if s2 Insufficient string length in n individual , Copy the entire s2, The rest are filled with empty characters until n individual .

I have questions when using these two functions ： If s or s1 The original string exists and the length of the string is longer than that of the new input s or s1 The length of the string is long , Then after entering or copying a new string ,s or s1 Is the original string replaced completely or partially . I tried it myself , A little discovery to share with you .

One 、gets function

chart 1
Operation diagram 1 Medium procedure , I asked s Entered in “Hello” The standard output function printf And string output functions puts Output a new string s, Get the picture 2 The results shown are .

   Both output functions output “Hello”, Do you use gets Function after entering a new string ,s The original existing string is completely replaced ？ I will printf("%s\n",s); Change it to printf("%c\n",s[8]); And enter the “Hello”, The program outputs a character w. A character array s From the Central Plains “I love nwpu” It doesn't seem to have been completely replaced . therefore , I use the single character output method to convert the character array s The elements in are output one by one .

chart 3

Operation diagram 3 Medium procedure , Input “Hello”, We get the graph 4 The results shown are .

chart 4

The program output “Hello nwpu”, From this we can draw a conclusion ： use gets Function after entering a new string ,s The original existing string is partially replaced . So why use printf("%s\n",s); or puts(s); When outputting, the program only outputs “Hello”？

It is worth noting that ,gets After the function input is completed , Automatically add a null character at the end of the input string . therefore ,“Hello” Of “o” There is an empty character after , chart 4 The output in is actually “Hello' \0' nwpu”,“o” And “n” Between them is an empty character and a space .puts(s); and printf("%s\n",s); When outputting a string , It ends as soon as the first null character is encountered , Whether or not it reaches the end of the array . So use printf("%s\n",s); or puts(s); When outputting, the program only outputs “Hello”.

From the above experiments, we can draw a more perfect conclusion ： use gets Function after entering a new string , A character array s The original existing string is partially replaced . A character array s The length of the original string to be replaced is the length of the new input string plus the length of an empty character .

Two 、strcpy、strncpy function ：

strcpy Functions and strncpy The experimental method of function and gets Function similar to , Make a brief description .

1、strcpy function ：

chart 5

Operation diagram 5 Medium procedure , Get the picture 6 The results shown are .strcpy Will s2 Empty characters in are also copied to s1 in , therefore s1 When outputting as a string ,printf The function ends the output when it encounters the first null character ; Character by character output s1 when ,“o” And “n” There is an empty character and a space between .

Conclusion ： use strcpy Function will s2 Copy the string in to s1 In the after , A character array s1 The original existing string is partially replaced . A character array s1 The length of the original string to be replaced is s2 The length of the string plus the length of an empty character .

2、strncpy function ：

chart 7

chart 8

Operation diagram 7 Medium procedure , Get the picture 8 The results shown are .strncpy take s2 Not more than n Copy characters of characters to s1 in , I used it here n by 2.strncpy Only will s2 The first two characters of the string in “h”、“e” Copied to the s1, It seems strncpy Empty characters are not automatically added , And it's s2 Middle front n Partial replacement of characters s1 In front of n Characters .

According to the experimental results ,strcpy and strncpy Empty characters are not automatically added at the end of the string . use strcpy Copying strings is possible due to s1 The storage space is small and the array is out of bounds , and strncpy You can avoid , however strncpy Copying may make s1 There is no empty character , for example ：

char s1[10];  strncpy(s1, " programming language",10);

strncpy usage ：

strncpy(s1,s2,sizeof(s1)-1);

s1[sizeof(s1)-1]='\0';

summary ： Whether it's for use gets Array of characters s Enter a string or use strcpy or strncpy Array of characters s1 Copy string from , All new strings replace the original strings .gets The function automatically adds an empty character at the end of the new string , and strcpy or strncpy Just to s2 Copied to the s1 in , Empty characters are not automatically added .